This invention relates to a method and appartus for filling molds with a binder material using internal gating techniques, particularly where the mold cavity is closely packed with a filler aggregate to leave relatively small interstices, so that it is difficult to infiltrate the binder without leaving unfilled voids between the aggregate.
This invention is of particular interest in connection with the manufacture of syntactic foam buoyancy modules of the type used to increase the buoyancy of under-sea apparatus, for instance, apparatus used in marine drilling rigs, or used in deep submersible vessels to provide buoyancy. Since these buoyancy modules are often submerged to great depths during their normal use, it is absolutely essential that the binder fully impregnate the interstices between the filler aggregate so that no air spaces remain, which air spaces contribute to the rapid deterioration or collapse of the modules which have been made defective by the presence of unfilled voids.
Such buoyancy modules are preferably made in molds whose cavities have been packed with aggregate comprising macrospheres which range in diameter from about 0.5 millimeters up to several inches or more. Often a mixture of different diameters of aggregate will be used to increase the packing factor of the aggregate. The packed molds are then filled with a curable resin binder which preferably takes the form of a syntactic foam comprising, itself, a composite material of hollow microspheres typically having diameters ranging around 60 microns. These microspheres are mixed with an uncured thermosetting polymeric resin which cures after injection into the mold to form a low density rigid matrix which fills the interstices between the macrospheres. This syntactic foam binder is often mixed under vacuum to insure that no free air is entrained therein. In order to achieve the lowest possible density for the matrix binder, the largest possible number of microspheres must be close-packed within the resin. It is of course not possible to achieve the highest ultimate degree of packing of microspheres by mixing them with the resin before filling the mold with it because the resin will become too viscous to flow as the number of microspheres approaches maximum. Therefore, packing of dry free-flowing microspheres into the mold containing the macrospheres will achieve the highest packing possible. Vibration and tamping techniques can be helpful in causing the microspheres and macrospheres to settle in an efficient array. The mold is closed, evacuated of free air, and neat matrix resin is then infiltrated into the voids between the aggregate filler spheres, often with the aid of pressurization of the source of resin to decrease the infiltration time. According to prior art techniques, the resin is sometimes introduced from a manifold through more than one periperal entry port in the mold where a larger buoyancy module is being molded.
However, even where more than one entry port is used to introduce the resin or syntactic foam binder, the infiltration of the aggregate is unnecessarily slowed by the fact that the binder must continuously flow a long path through the filler aggregate from an opening to the farthest air bleeder outlet. This has two very serious practical disadvantages. One is the slowness of infiltration due to long and tortuous paths of travel for the binder, requiring the use of relatively high pumping pressures. The other disadvantage results from the use of high pumping pressures which creates a strong tendency of the entering binder to push the aggregate spheres away from the entry port by bunching them more tightly in the far portions of the mold. This latter effect creates a zone near the entry ports where there is no aggregate remaining.
In addition the frictional pressure of the filler as it is pumped between the macrospheres tends to crush some of them, which crushing is aided by the fact that the contacts between spheres are virtually point-contacts where the unit-area pressures are very high.
When aggregate spheres are crushed, free air voids appear in the finished buoyancy modules and these voids contribute to failures of the modules under the extremes of deep ocean water pressures. A module having multiple free air voids fails by having the sea water pressure crush its way into a void near the surface of the module, and then travel progressively from one void to the next into the interior of the module. Pockets of submersion-induced pressures can thus cause submerged modules to implode.
According to the usual techniques for manufacturing buoyancy modules, the binder foam or neat resin is forced to infiltrate the interstices between the aggregate filler using external pumping pressure, or evacuation of the mold, or both. The binder is introduced at one or more entry ports through the periphery of the mold. Depending on the size and shape of the mold, upon the particle sizes and the size-distribution in the mold, and upon the viscosity of the binder, the infiltration process might take several hours using prior art techniques. The use of vibrators can increase the flow rate within limitations, and the total infiltration time can be further reduced by using multiple entry ports. In some instances the prior art has used external gating wherein the foam enters the mold along a series of peripheral linear ports rather than at discrete holes, and this technique has been used in combination with evacuation of air from the mold. This peripheral linear-port technique resembles somewhat the metal mold filling apparatus shown in U.S. Pat. No. 2,446,235 to Hawk et al.
U.S. Pat. No. 1,579,743 to Warlow Jr., and U.S. Pat. No. 3,598,175 to Olsson et al both show a mold housing having multiple individual identical molds arrayed in the housing and all being fed from a central gate tube having holes opposite each of the individual molds to fill them. However, highly liquid metal is being introduced through this gating, which seeks to fill the molds quickly before the metal cools, and which requires filling all the individual molds approximately simultaneously. There is no aggregate filler occupying any of the molds shown. Moreover, filling pressures need not be high because there are no tortuous interstitial spaces to be filled and the filler is not viscous.